A. Field of the Invention
This invention relates generally to botanical measuring systems, and more particularly to a method and apparatus for electrically measuring the combined water status of a plant and its root environment by measuring the long term variations of the direct current electrical potential biologically developed between a point on the body of the plant and its root environment.
B. Prior Art
Prior art techniques for measuring the water status of a plant and its root environment consist essentially of physically measuring two parameters, namely, water potential and water content. The water potential is the chemical potential of the water in a predefined system, and is defined as the partial derivative of the Gibbs free energy of the system with respect to a change in the molar concentration of water in the system. The water potential that is measured by prior art methods is displayed as a change in the water potential with respect to a predefined reference.
The water potential arises from three physical sources. It is a measure of the change in energy per mole of water caused by solutes in the system, the energy arising from the interaction at a phase interface between solids and the water in the system, and the energy arising when the system is pressurized above ambient pressure.
The second component of the water status of a plant and its environment is simply water content which may be measured by weighing a soil sample, heating the sample to drive off the water, and then weighing the sample again.
Both of the above measurements for determining water status are purely physical measurements which may be damaging to the plant, and which are very time consuming. Furthermore, they do not assess the water continuum which exits between the root environment and the plant structure itself. Accordingly, it is desirable to provide a relatively simple method for electrically measuring the combined water status of a plant and its root environment. Such a technique would eliminate the need for making two separate distinct physical measurements and would provide a direct indication of the combined water status.
Electrical measuring devices have been used to measure the resistance of plants. Such devices generally impress an electrical potential on the plant or soil and measure the resulting current flow to obtain an indication of the conductivity. The physiological changes resulting from the impressed potential are also observed. The propagation of short term action potentials along the stems of plants has also been studied. One such study is described in Barbara G. Pickard, "Action Potentials In Higher Plants", Botanical Review, Vol. 39, No. 2, April-June 1973. The above mentioned article describes the use of electrical techniques for measuring short term variations in action potentials between points on a stem in response to external stimuli, but does not contemplate the measurement of slowly varying changes in potential between the root environment and a point on the body of a plant to provide an indication of the combined water status of the plant and its root environment.